The most common topology of a satellite network is the star topology, in which a central hub broadcasts toward a plurality of remote terminals (such as very small aperture terminals, or VSATs) via a forward time-domain multiplexed (TDM) carrier, and the remote terminals communicate with the hub by return channel bursts. There are many common multiple access schemes, the choice of which involves a dilemma and a well-known trade-off. Contention-based random access (CRA) solutions, such as Slotted Aloha, tend to have short response-time per transaction. However, the throughput is limited theoretically to about 36% utilization of the available channel. Moreover, in order to achieve acceptable average delays (that are dominated by collisions and the need for re-transmissions), the practical load should be about 30%. A more bandwidth efficient access scheme would provide a collision-free approach. A simple fixed time-domain multiple-access (TDMA) scheme, where slot allocations are pre-determined, is not efficient in the traffic regime where the terminal return link needs are highly dynamic and variable.
A more popular approach is a “reservation-based” solution, in which the hub allocates time slots and frequencies to remote terminals according to the momentary needs of the remote terminals. This solution is also referred as a “reservation” system. In a reservation system, the remote terminals each transmit over a return control channel a “request for allocation” message to the hub whenever there is a need to transfer data to the hub. However, the repeated transmission of request for allocation messages imposes significant delay and added transmission overhead. A good design would try to keep the request for allocation messages very short on a special short-slots structure.
Multiple-access VSAT networks often involve complicated time and frequency synchronization aimed to align the burst transmissions into the pre-designed frequency and time slot scheme. It is particularly complicated to align the first transmission of a remote terminal when, for example, the remote terminal has just been powered up. Traditionally, synchronization for both burst-slot timing and frame timing is accomplished via synchronization time-stamps that are sent via the forward channel. This involves some over-head in the Forward channel. Moreover, it involves costly and complicated hardware in the central hub, for accurate insertions and stamping of these time stamps within the forward channel.
Moreover, existing reservation networks cannot deal with initial frequency errors that are more than a fraction of channel frequency spacing, and cannot tolerate burst-timing errors that are larger than slot time guard bands. To allow for proper timing, the remote terminals are typically provided with information about their own geographic location and satellite location (to determine the distance between the terminal and the satellite), as well as time-stamps. However, the geographic information requirement makes it difficult to provide for mobile remote stations, and makes setup more difficult and time consuming.
In addition, because there are typically relatively few satellites and hubs in a satellite network as compared with remote terminals, and because many remote terminals are intended to be sold to individuals, remote terminals are often manufactured with less expensive electronics than in satellites and hubs. For example, each of the satellites, hubs, and remote terminals has circuitry that generates its outgoing carrier frequency or frequencies. Although such circuitry may be extremely accurate in the satellites and hubs, the less expensive carrier frequency generation circuitry in the remote terminals may be subject to drift and/or other frequency error. Such errors can be problematic in that the various carrier frequencies used in a satellite network are often close together. Any substantial frequency error can result in miscommunication.